Spetroscopic pH measurement using optimized mixtures of reagents to extend measurement range

Abstract

An indicator mixture that allows pH measurement over a broader range of pH or to a higher accuracy than available using conventional spectroscopic techniques. In particular, the mixture of the present invention is comprised of two or more reagents such that when combined, the reagent mixture is capable of either detecting: (1) a pH range broader or more accurate than that the reagents individually, or (2) pH more accurately than the reagents individually. Also disclosed are methods of making and using the mixture.

1. A method of identifying one or more sets of reagents wherein the one or more sets of reagents is used for determining the pH of a sample, the method comprising:
 a. identifying a target pH measurement range of the sample and a target pH accuracy; 
 b. identifying a plurality of reagents based on known reagent thermodynamic acid dissociation constants and spectral characteristics such that their combination is expected to satisfy the targeted pH range of the sample; 
 c. identifying one or more spectral channels to make the pH measurement wherein the reagents have elevated optical densities; 
 d. mixing known relative concentrations of two or more reagents of the plurality of reagents to create one or more sets of reagents; 
 e. characterizing at least one reagent set of the one or more sets of reagents wherein characterizing includes developing a relationship between optical density ratio and pH based on the thermodynamic acid dissociation constants of the reagents, relative concentrations of the reagents, and one or more spectral channels; 
 f. identifying the spectroscopic noise of a spectral analyzer to be used for the pH measurement; 
 g. performing an error analysis for at least one reagent set of the one or more sets of reagents wherein the error analysis comprises developing a relationship between standard deviation in pH and spectroscopic noise in optical density based on the relation developed in step (e); 
 h. optimizing the one or more sets of reagents with an optimization algorithm based on the developed relationship in step (g) to satisfy the target pH accuracy over the target pH measurement range of the sample and determine one of optimum values for relative concentrations or spectral channels or both; and 
 i. characterizing the one or more optimized reagent set. 
 
     
     
       2. The method of  claim 1 , further comprising the step of characterizing the at least one of the one or more sets of optimized reagents by developing a relationship between optical density ratio and pH using the optimized parameters. 
     
     
       3. The method of  claim 2 , further comprising checking for uniqueness of the correlation between optical density ratio and pH. 
     
     
       4. The method of  claim 2 , wherein the one or more optimized sets of reagents is suitable for use in a downhole environment, the method further comprising:
 a. storing the optimized set of reagents in a reagent container coupled to a fluids analyzer, wherein said reagent mixture is capable of detecting a pH range broader than each reagent individually; 
 b. positioning the fluids analyzer downhole; 
 c. drawing formation fluid into the fluids analyzer; 
 d. mixing formation fluid with the reagent mixture; and 
 e. analyzing the optical density of said mixture of formation fluid and reagent mixture in one or more spectral channels to determine the pH of the formation fluid. 
 
     
     
       5. The method of  claim 4 , wherein said pH is determined to an accuracy of 0.5 pH units or less. 
     
     
       6. The method of  claim 4 , wherein analyzing the optical density is performed using two or more channels. 
     
     
       7. The method of  claim 6 , further comprising analyzing the optical density ratio. 
     
     
       8. The method of  claim 4 , comprising obtaining optical measurements in three or more channels and performing a regression analysis using said optical measurements. 
     
     
       9. The method of  claim 1 , further comprising optimizing absolute concentration of the one or more sets of reagents. 
     
     
       10. The method of  claim 1 , wherein the reagents show similar direction of spectral shift with changes in pH. 
     
     
       11. The method of  claim 1 , wherein identifying the spectroscopic noise includes one of signal to noise ratio of spectral signal or standard deviation and the optical density.